Yes, casual hate. Anything that doesn't give everything in the game to those that never play is casual hate.
I'd argue that if Blizzard caved and gave the Mage Tower appearances that were said to be exclusive again to those that didn't do them during the many month window they were available, many more would complain.
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While I'm not exactly a terrible player, I got all 36 of the appearances, many of which were on classes I had little to no experience on at all, with (for the final patch) very average gear. 'Incredibly difficult' might be a tad exaggerated.
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Scaling content is never going to be able to ensure the difficulty stays the same. Time exclusive rewards also entices people to play while they're available. If they're always available there's no reason to do them while they're current, and people end up playing less as a whole.